System and method for assessing effective delivery of ablation therapy

ABSTRACT

A system and method for assessing effective delivery of ablation therapy to a tissue in a body is provided. A three-dimensional anatomical map of the tissue is generated and displayed with the map defining a corresponding volume. An index is generated corresponding to a location within the volume with the index indicative of a state of ablation therapy at the location. The index may be derived from one or more factors such as the duration an ablation electrode is present at the location, the amount of energy provided, the degree of electrical coupling between an ablation electrode and the tissue at the location and temperature. A visual characteristic (e.g., color intensity) of a portion of the anatomical map corresponding to the location is then altered responsive to the index.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/195,249, filed 19 Nov. 2018 (the '249 application), which is adivisional of U.S. application Ser. No. 14/872,208, filed 1 Oct. 2015(the '208 application) now U.S. Pat. No. 10,130,419, which is acontinuation of U.S. application Ser. No. 13/889,732, filed 8 May 2013(the '732 application), now U.S. Pat. No. 9,173,611, which is acontinuation of U.S. application Ser. No. 12/622,626, filed 20 Nov. 2009(the '626 application), now U.S. Pat. No. 8,454,589. The '249application, the '208 application, the '732 application, and the '626application are all hereby incorporated by reference as though fully setforth herein.

BACKGROUND OF THE INVENTION a. Field of the Invention

This invention relates to a system and method for assessing theeffective delivery of ablation therapy to tissue in a body. Inparticular, the instant invention relates to a system and method forgenerating and displaying an anatomical map of the tissue and altering avisual characteristic of a portion of the map responsive to an indexindicative of the state of ablation therapy.

b. Background Art

Ablation catheters are commonly used to create tissue necrosis incardiac tissue to correct conditions such as atrial arrhythmia(including, but not limited to, ectopic atrial tachycardia, atrialfibrillation, and atrial flutter). Arrhythmia can create a variety ofdangerous conditions including irregular heart rates, loss ofsynchronous atrioventricular contractions and stasis of blood flow whichcan lead to a variety of ailments and even death. It is believed thatthe primary cause of atrial arrhythmia is stray electrical signalswithin the left or right atrium of the heart. The ablation catheterimparts ablative energy (e.g., radiofrequency energy, cryoablation,lasers, chemicals, high-intensity focused ultrasound, etc.) to cardiactissue to create a lesion in the cardiac tissue. This lesion disruptsundesirable electrical pathways and thereby limits or prevents strayelectrical signals that lead to arrhythmias.

Electroanatomical mapping systems are frequently used during cardiacablation procedures to generate a visual representation, or anatomicalmap, of the endocardial surface. This visual representation permits aclinician to track the locations at which ablation therapy is providedand to view the cardiac structure and ablative lesions from a variety ofangles. Tracking ablation delivery sites is important because it isoften necessary to achieve a contiguous line of necrosis to disruptundesirable electrical pathways in the tissue-despite the fact thatablation therapy often involves multiple applications of energy atdiscrete locations.

Conventional anatomical mapping systems used during ablation proceduresrequire the clinician to manually mark treatment regions on theanatomical map. In particular, the clinician provides an input through aconventional input device (e.g., a mouse, keyboard, etc.) to indicatethe location or locations on the map at which ablation lesions have beencreated. This subjective marking occurs despite numerous variabilitiesincluding movement of the ablation catheter and heart due to cardiaccontractions, ventilation and movements of the clinician and variationin the application of ablative energy resulting, for example, fromchanging temperature or impedance levels. As a result, there may not bea strong correlation between lesions marked on the anatomical map by theclinician and the effective delivery of ablation therapy. Clinicianstherefore frequently engage in repeated and relatively time consumingelectrophysiologic mapping procedures to identify locations whereadditional ablation therapy is required and then provide additionalablative energy to those locations-often in a repeated cycle. Theseproblems are exacerbated for those clinicians who engage in procedureswhereby the ablation catheter is intentionally moved (or “dragged”)while generating ablative energy. This type of procedure can help reducecollateral tissue damage and is more time efficient, but increases theodds that manual marking of lesion sites will be inaccurate.

The inventors herein have recognized a need for a system and method forassessing the effective delivery of ablation therapy to tissue in a bodythat will minimize and/or eliminate one or more of the above-identifieddeficiencies.

BRIEF SUMMARY OF THE INVENTION

It is desirable to provide a system and method for assessing theeffective delivery of ablation therapy to tissue in a body. Inparticular, it is desirable to be able to alter an anatomical map in anobjective fashion to indicate the effective delivery of ablation therapyto tissue.

A system for assessing the effective delivery of ablation therapy totissue in a body in accordance with one embodiment of the presentteachings includes an electronic control unit configured to generate athree-dimensional anatomical map of the tissue. The map defines acorresponding volume. The system further includes a display configuredto display the anatomical map. The electronic control unit is furtherconfigured to generate an index corresponding to a location within thevolume. The index is indicative of a state of ablation therapy at thelocation. In accordance with various embodiments of the presentteachings, the index may be determined responsive to various factorsincluding, for example, the duration of time during which ablationenergy is provided at the location, the amount of ablation energydelivered by the ablation catheter at the location, the temperature ator near the location and the degree of contact or electrical couplingbetween an ablation electrode and the tissue. The electronic controlunit is further configured to alter, responsive to the index, a visualcharacteristic of a portion of the anatomical map corresponding to thelocation. In one embodiment according to the present teachings, thevolume is divided into a plurality of voxels (volume elements) and thealtered portion of the anatomical map comprises one of the voxelscontaining the location. In another embodiment according to the presentteachings, the anatomical map defines a surface and the altered portionof the map comprises a part of the surface that is altered responsive tothe index and a distance between the location and the part of thesurface.

A method for assessing the effective delivery of ablation therapy totissue in a body in accordance with another embodiment of the presentteachings includes the step of generating a three-dimensional anatomicalmap of the tissue. The map defines a corresponding volume. The methodfurther includes the step of displaying the anatomical map. The methodfurther includes the step of generating an index corresponding to alocation within the volume, the index indicative of a state of ablationtherapy at the location. The method further includes the step ofaltering, responsive to the index, a visual characteristic of a portionof the anatomical map corresponding to the location.

The above-described system and method are advantageous because theyprovide a less subjective and more accurate map of ablation lesions thanconventional systems. The use of an index based on one or more factorsor predictors indicative of the state of ablative therapy provides amore objective assessment of the effective delivery of ablation therapywhile reducing the impact of variabilities such as catheter or tissuemovement or changes in operating conditions. As a result, clinicians canassess the effectiveness of ablation therapy with greater confidence andreduce or eliminate the need for electrophysiologic mapping proceduresand repeated ablation.

The foregoing and other aspects, features, details, utilities andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagrammatic view of a system in accordance with the presentteachings.

FIG. 2 is a simplified schematic diagram illustrating how a degree ofelectrical coupling between an electrode and tissue may be determined.

FIG. 3 is a screen display illustrating one embodiment of an anatomicalmap altered in accordance with the present teachings.

FIG. 4 is a screen display illustrating another embodiment of ananatomical map altered in accordance with the present teachings.

FIG. 5 is a series of timing diagrams illustrating variations over timein several factors that may be used in an index to indicate effectivedelivery of ablation therapy.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring now to the drawings wherein like reference numerals are usedto identify identical components in the various views, FIG. 1illustrates one embodiment of a system 10 for one or more diagnostic andtherapeutic functions including components providing an assessment ofthe effective delivery of ablation therapy to tissue 12 in a body 14. Inthe illustrated embodiment, tissue 12 comprises heart or cardiac tissue.It should be understood, however, that the present invention may be usedto assess effective delivery of ablation therapy to a variety of bodytissues. System 10 may include an ablation catheter 16, patch electrodes18, 20, 22, an ablation generator 24, a tissue sensing circuit 26, anelectrophysiology (EP) monitor 28 and a system 30 for visualization,mapping and navigation of internal body structures which may include anelectronic control unit 32 in accordance with the present invention, adisplay device 34 and an input/output (I/O) device 36 among othercomponents.

Catheter 16 is provided for examination, diagnosis and treatment ofinternal body tissues such as tissue 12. In accordance with oneembodiment of the present teachings, catheter 16 comprises an ablationcatheter. Catheter 16 may comprise, for example, an ablation catheter ofthe type sold commercially by St. Jude Medical, Inc. under the “SAFIRE”or “COOL PATH” trademarks and having a four millimeter tip that may bedeflected in one direction (uni-directional) or multiple directions(bi-directional). In accordance with one embodiment of the presentteachings, catheter 16 comprises an irrigated radio-frequency (RF)ablation catheter. It should be understood, however, that the presentinvention can be implemented and practiced regardless of the type ofablation energy provided (e.g., cryoablation, ultrasound, etc.).Catheter 16 is connected to a fluid source 38 having a biocompatiblefluid such as saline through a pump 40 (which may comprise, for example,a fixed rate roller pump or variable volume syringe pump with a gravityfeed supply from fluid source 38 as shown) for irrigation. Catheter 16is also electrically connected to ablation generator 24 for delivery ofRF energy. Catheter 16 may include a cable connector or interface 42, ahandle 44, a shaft 46 having a proximal end 48 and a distal 50 end (asused herein, “proximal” refers to a direction toward the end of thecatheter near the clinician, and “distal” refers to a direction awayfrom the clinician and (generally) inside the body of a patient) and oneor more electrodes 52, 54, 56. Catheter 16 may also include otherconventional components not illustrated herein such as a temperaturesensor, additional electrodes, and corresponding conductors or leads.

Connector 42 provides mechanical, fluid and electrical connection(s) forcables 58, 60 extending from pump 40 and ablation generator 24.Connector 42 is conventional in the art and is disposed at a proximalend of catheter 16.

Handle 44 provides a location for the clinician to hold catheter 16 andmay further provide means for steering or guiding shaft 46 within body14. For example, handle 44 may include means to change the length of aguidewire extending through catheter 14 to distal end 50 of shaft 46 tosteer shaft 46. Handle 44 is also conventional in the art and it will beunderstood that the construction of handle 44 may vary.

Shaft 46 is an elongated, tubular, flexible member configured formovement within body 14. Shaft 46 support electrodes 52, 54, 56associated conductors, and possibly additional electronics used forsignal processing or conditioning. Shaft 46 may also permit transport,delivery and/or removal of fluids (including irrigation fluids andbodily fluids), medicines, and/or surgical tools or instruments. Shaft46 may be made from conventional materials such as polyurethane anddefine one or more lumens configured to house and/or transportelectrical conductors, fluids or surgical tools. Shaft 46 may beintroduced into a blood vessel or other structure within body 14 througha conventional introducer. Shaft 46 may then be steered or guidedthrough body 14 to a desired location such as tissue 12 with guide wiresor other means known in the art.

Electrodes 52, 54, 46 are provided for a variety of diagnostic andtherapeutic purposes including, for example, electrophysiologicalstudies, catheter identification and location, pacing, cardiac mappingand ablation. In the illustrated embodiment, catheter 16 includes anablation tip electrode 52 at distal end 50 of shaft 46 and a pair ofring electrodes 54, 56. It should be understood, however, that thenumber, orientation and purpose of electrodes 52, 54, 56 may vary.

Patch electrodes 18, 20, 22 provide RF or navigational signal injectionpaths and/or are used to sense electrical potentials. Electrodes 18, 20,22 may also have additional purposes such as the generation of anelectromechanical map. Electrodes 18, 20, 22 can be made from flexible,electrically conductive material and be configured for affixation tobody 14 such that electrodes 18, 20, 22 are in electrical contact withthe patient's skin. Alternatively, electrodes 18, 20, 22 can be part ofa pad or support placed under the patient. Electrode 18 may function asan RF indifferent/dispersive return for the RF ablation signal.Electrodes 20, 22 may function as returns for the RF ablation signalsource and/or an excitation signal generated by tissue sensing circuit26 as described in greater detail hereinbelow. Electrodes 20, 22 arepreferably spaced relatively far apart for a purpose describedhereinbelow. In the illustrated embodiment, electrodes 20, 22 arelocated on the medial aspect of the left leg and the dorsal aspect ofthe neck. Electrodes 20, 22 may alternatively be located on the frontand back of the torso or in other conventional orientations.

Ablation generator 24 generates, delivers and controls RF energy used byablation catheter 16. Generator 24 is conventional in the art and maycomprise the commercially available unit sold under the model numberIBI-1500T RF Cardiac Ablation Generator, available from IrvineBiomedical, Inc. Generator 24 includes an RF ablation signal source 62configured to generate an ablation signal that is output across a pairof source connectors: a positive polarity connector SOURCE (+) which mayconnect to tip electrode 52; and a negative polarity connector SOURCE(−)which may be electrically connected by conductors or lead wires to oneof patch electrodes 18, 20, 22 (see FIG. 2). It should be understoodthat the term connectors as used herein does not imply a particular typeof physical interface mechanism, but is rather broadly contemplated torepresent one or more electrical nodes. Source 62 is configured togenerate a signal at a predetermined frequency in accordance with one ormore user specified parameters (e.g., power, time, etc.) and under thecontrol of various feedback sensing and control circuitry as is know inthe art. Source 62 may generate a signal, for example, with a frequencyof about 450 kHz or greater. Generator 24 may also monitor variousparameters associated with the ablation procedure including impedance,the temperature at the tip of the catheter, ablation energy and theposition of the catheter and provide feedback to EP monitor 28 andsystem 30.

Tissue sensing circuit 26 provides a means, such as tissue sensingsignal source 64, for generating an excitation signal used in impedancemeasurements and means, such as complex impedance sensor 66, forresolving the detected impedance into its component parts. Signal source64 is configured to generate an excitation signal across sourceconnectors SOURCE (+) and SOURCE (−) (See FIG. 2). Source 64 may outputa signal having a frequency within a range from about 1 kHz to over 500kHz, more preferably within a range of about 2 kHz to 200 kHz, and evenmore preferably about 20 kHz. In one embodiment, the excitation signalis a constant current signal, preferably in the range of between 20-200μA, and more preferably about 100 μA. As discussed below, the constantcurrent AC excitation signal generated by source 64 is configured todevelop a corresponding AC response voltage signal that is dependent onthe complex impedance of tissue 12 and is sensed by complex impedancesensor 66. Sensor 66 resolves the complex impedance into its componentparts (i.e., the resistance (R) and reactance (X) or the impedancemagnitude (|Z|) and phase angle (∠Z or ϕ)). Sensor 66 may includeconventional filters (e.g., bandpass filters) to block frequencies thatare not of interest, but permit appropriate frequencies, such as theexcitation frequency, to pass as well as conventional signal processingsoftware used to obtain the component parts of the measured compleximpedance.

It should be understood that the excitation signal from source 64 mayalternatively be an AC voltage signal where the response signalcomprises an AC current signal. It should also be appreciated that theexcitation signal frequency is preferably outside of the frequency rangeof the RF ablation signal, which allows the complex impedance sensor 66to more readily distinguish the two signals, and facilitates filteringand subsequent processing of the AC response voltage signal. Theexcitation signal frequency is also preferably outside the frequencyrange of conventionally expected electrogram (EGM) signals in thefrequency range of 0.05-1 kHz. Thus, in summary, the excitation signalpreferably has a frequency that is preferably above the typical EGMsignal frequencies and below the typical RF ablation signal frequencies.

Circuit 26 is also connected, for a purpose described hereinbelow,across a pair of sense connectors: a positive polarity connector SENSE(+) which may connect to tip electrode 52; and a negative polarityconnector SENSE (−) which may be electrically connected to one of patchelectrodes 18, 20, 22 (see FIG. 2; note, however, that the connectorSENSE (−) should be connected to a different electrode of electrodes 18,20, 22 relative to the connector SOURCE (−) as discussed below) oranother electrode 54, 56 on catheter 16, such as ring electrode 54 asdescribed in commonly assigned U.S. patent application Ser. No.11/966,232 filed on Dec. 28, 2007 and titled “System and Method forMeasurement of an Impedance Using a Catheter Such as an AblationCatheter,” the entire disclosure of which is incorporated herein byreference. It should again be understood that the term connectors asused herein does not imply a particular type of physical interfacemechanism, but is rather broadly contemplated to represent one or moreelectrical nodes.

Referring now to FIG. 2, connectors SOURCE (+), SOURCE (−), SENSE (+)and SENSE (−) form a three terminal arrangement permitting measurementof the complex impedance at the interface of tip electrode 52 and tissue12. Complex impedance can be expressed in rectangular coordinates as setforth in equation (1):

Z=R+jX  (1)

where R is the resistance component (expressed in ohms); and X is areactance component (also expressed in ohms). Complex impedance can alsobe expressed in polar coordinates as set forth in equation (2):

Z=r·e ^(jθ) =|Z|·e ^(j∠Z)  (2)

where |Z| is the magnitude of the complex impedance (expressed in ohms)and ∠Z=θ is the phase angle expressed in radians. Alternatively, thephase angle may be expressed in terms of degrees where

$\phi = {\left( \frac{180}{\pi} \right){\theta.}}$

Throughout the remainder of this specification, phase angle will bepreferably referenced in terms of degrees. The three terminals comprise:(1) a first terminal designated “A-Catheter Tip” which is the tipelectrode 52; (2) a second terminal designated “B-Patch 1” such assource return patch electrode 22; and (3) a third terminal designated“C-Patch 2” such as the sense return patch electrode 20. In addition tothe ablation (power) signal generated by source 62 of ablation generator24, the excitation signal generated by source 64 in tissue sensingcircuit 26 is also applied across the source connectors (SOURCE (+),SOURCE (−)) for the purpose of inducing a response signal with respectto the load that can be measured and which depends on the compleximpedance. As described above, in one embodiment, a 20 kHz, 100 μA ACconstant current signal is sourced along the path 68, as illustrated,from one connector (SOURCE (+), starting at node A) through the commonnode (node D) to a return patch electrode (SOURCE (−), node B). Thecomplex impedance sensor 66 is coupled to the sense connectors (SENSE(+), SENSE (−)), and is configured to determine the impedance across thepath 70. For the constant current excitation signal of a linear circuit,the impedance will be proportional to the observed voltage developedacross SENSE (+)/SENSE(−), in accordance with Ohm's Law: Z=V/I. Becausevoltage sensing is nearly ideal, the current flows through the path 68only, so the current through path 70 (node D to node C) due to theexcitation signal is effectively zero. Accordingly, when measuring thevoltage along path 70, the only voltage observed will be where the twopaths intersect (i.e. from node A to node D). Depending on the degree ofseparation of the two patch electrodes (i.e., those forming nodes B andC), an ever-increasing focus will be placed on the tissue volume nearestthe tip electrode 52. If the patch electrodes are physically close toeach other, the circuit pathways between the catheter tip electrode 52and the patch electrodes will overlap significantly and impedancemeasured at the common node (i.e., node D) will reflect impedances notonly at the interface of the catheter electrode 52 and tissue 12, butalso other impedances between tissue 12 and the surface of body 14. Asthe patch electrodes are moved further part, the amount of overlap inthe circuit paths decreases and impedance measured at the common node isonly at or near the tip electrode 52 of catheter 16.

Referring again to FIG. 1, EP monitor 28 is provided to displayelectrophysiology data including, for example, an electrogram. Monitor28 is conventional in the art and may comprise an LCD or CRT monitor oranother conventional monitor. Monitor 28 may receive inputs fromablation generator 24 as well as other conventional EP lab componentsnot shown in the illustrated embodiment.

System 30 is provided for visualization, mapping and navigation ofinternal body structures. System 30 may comprise the system having themodel name EnSite™ NavX™ and commercially available from St. JudeMedical., Inc. and as generally shown with reference to commonlyassigned U.S. Pat. No. 7,263,397 titled “Method and Apparatus forCatheter Navigation and Location and Mapping in the Heart,” the entiredisclosure of which is incorporated herein by reference. Alternatively,system 30 may comprise the system sold under the name Carto™ andcommercially available from Biosense Webster, Inc. or a magneticlocation system such as the system sold under the name gMPS andcommercially available from Mediguide Ltd. and as generally shown withreference to U.S. Pat. No. 7,386,339 entitled “Medical Imaging andNavigation System”, the entire disclosure of which is incorporatedherein by reference. System 30 may include electronic control unit (ECU)32, display device 34 and I/O device 36 among other components.

ECU 32 is provided to generate anatomical maps of tissue 12, to generatean index indicative of a state of ablation therapy at a location withina volume defined by an anatomical map and to alter a visualcharacteristic of at least a portion of an anatomical map correspondingto the location. ECU 32 preferably comprises a programmablemicroprocessor or microcontroller, but may alternatively comprise anapplication specific integrated circuit (ASIC). ECU 32 may include acentral processing unit (CPU) and an input/output (I/O) interfacethrough which ECU 32 may receive a plurality of input signals includingsignals from sensor 66 of tissue sensing circuit 26, from ablationgenerator 24 and from I/O device 36 and generate a plurality of outputsignals including those used to control display device 34. In accordancewith one aspect of the present invention, ECU 32 may be programmed witha computer program (i.e., software) encoded on a computer storage mediumfor performing one or more of the above functions.

Referring to FIG. 3, ECU 32 generates a three-dimensional anatomical map72 for illustration on display device 34 in a conventional manner. Aplurality of electrodes at one end of catheter 16 or a conventionalmapping catheter may be moved within the heart chambers by the clinicianwhile the heart is beating. The locations of the catheter electrodes aremeasured, for example, using patch electrodes 18, 20, 22 or anothersensor (e.g., a magnetic sensor (not shown)) and stored by ECU 32 as a“cloud” of points. A conventional algorithm such as a convex hullalgorithm is used to construct a surface around the cloud. The mostexterior points are used to create a “shell” representing the shape ofthe heart. Additional sampling and smoothing operations are thenperformed to generate the anatomical map 72 shown in FIG. 3. A moredetailed explanation of the mapping process is described in theabove-referenced U.S. Pat. No. 7,263,397 titled “Method and Apparatusfor Catheter Navigation and Location and Mapping in the Heart,” theentire disclosure of which is incorporated herein by reference. Itshould also be understood that ECU 32 could generate map 72 based on areal-time image such as an ultrasound image or fluoroscopic image or apre-acquired image (e.g., an image generated by an MRI, CT, ultrasoundor fluoroscopic system).

Three-dimensional map 72 defines a volume 74 comprising the space withinits surface boundaries. ECU 32 is further configured generate an index(CAI) indicative of a state of ablation therapy at a location, such aslocation 76, within the volume 74. The index is derived from one or moreof a plurality of factors indicative of delivery of ablative therapy totissue 12. These factors may include a duration of time during whichablation tip electrode 52 is present at the location or ablation therapyis provided at the location. As noted above, the location of electrode52 can be tracked by ECU 32 by reading voltage levels on electrodes 18,20, 22. Another factor may be the amount of ablation energy provided atthe location. This information can be obtained from ablation generator24 as a measurement of RF power in the case of radio-frequency ablationand will vary based on programmed instructions for delivery of energyand feedback control such as temperature measurements (to avoid charringor steam pops) and impedance measurements (to minimize coagulumformation). Another factor is the temperature proximate the distal end50 of catheter 16 where tip electrode 52 is located. The temperature canbe measured using a conventional temperature sensor (not shown) on thedistal end of shaft 46 of catheter 16. Other exemplary factors includean impedance measurement or a change in amplitude in an electrogram bothas measured by generator 24. Additional factors may include the flow ofirrigation fluid in catheter 16 as measured by a conventional flowsensor or meter (not shown) or the orientation of electrode 52 relativeto tissue 12 which could be measured by a force vector sensor (notshown) on catheter 16 or through impedance measurements. Another factorindicative of the state of ablative therapy is contact pressure betweenelectrode 52 and tissue 12 which can be measured through a conventionalpressure sensor (not shown).

Another factor that is indicative of effective delivery of ablationtherapy and that may be used in the index CAI is the degree of coupling,and particularly electrical coupling, between electrode 52 and tissue12. As discussed in greater detail hereinabove, ECU 32 can acquire oneor more values for two component parts of the complex impedance fromsignals generated by sensor 66 of tissue sensing circuit 26 (i.e., theresistance (R) and reactance (X) or the impedance magnitude (|Z|) andphase angle (ϕ) or any combination of the foregoing or derivatives orfunctional equivalents thereof). ECU 32 may combine values for the twocomponents into a single coupling index (ECI) that provides a measure ofthe degree of coupling between electrode 52 and tissue 12 and, inparticular, the degree of electrical coupling between electrode 52 andtissue 12. This process and index are described more fully in commonlyassigned U.S. patent application Ser. No. 12/253,637 titled “System andMethod for Assessing Coupling Between an Electrode and Tissue” filedOct. 17, 2008, the entire disclosure of which is incorporated herein byreference. As discussed in that application it should be understood thatcoefficients, offsets and values within the equation for the couplingindex may vary depending on among other things, the desired level orpredictability, the species being treated and disease states. Becauseimpedance measurements are also influenced by the design of catheter 16connection cables 58, 60 or other factors, the coupling index ECI maypreferably comprise a flexible equation in which coefficients andoffsets are variable in response to design parameters associated withcatheter 16. Catheter 16 may include a memory such as an EEPROM thatstores numerical values for the coefficients and offsets or stores amemory address for accessing the numerical values in another memorylocation (either in the catheter EEPROM or in another memory). ECU 32may retrieve these values or addresses directly or indirectly from thememory and modify the coupling index ECI accordingly. The physicalstructure of the patient is another factor that may influence impedancemeasurements and the coupling index. Therefore, ECU 32 may also beconfigured to offset or normalize the coupling index (e.g., by adjustingcoefficients or offsets within the index ECI) responsive to an initialmeasurement of impedance or another parameter in a particular patient.In addition, it may be beneficial to obtain and average values for thecoupling index ECI responsive to excitation signals generated by source64 at multiple different frequencies. It should be understood that whilethe coupling index ECI provides a measure of the degree of electricalcoupling between electrode 52 and tissue 12, measures of the degree ofphysical coupling can also be used as a factor in the index CAIincluding, for example, contact force or contact pressure as notedabove.

ECU 32 may maintain, read from and update a data structure containingvalues for the index CAI and the factors (e.g., RF power, temperature,ECI, etc.) used to determine the index CAI with each index value (andthe associated factor values) corresponding to either a discretelocation, such as location 76, within volume 74 or a plurality oflocations as discussed hereinbelow. ECU 32 continuously calculates andmaintains the value of the index CAI for any particular location orgroup of locations over time such that the index CAI represents anassessment of the instantaneous and cumulative values for the variousfactors that make up the index. The index CAI may therefore berepresented by the following equation:

${{CAI}\left( {x,y,z,t} \right)} = {\int\limits_{t_{0}}^{t}{{f\left( {x,y,z,t} \right)}{dt}}}$

where x, y, and z represent coordinates for a location 76, t representstime and f(x,y,z,t) is a function g of one or more of theabove-described factors. For example, in one embodiment of theinvention, the index CAI may be derived from the following function:

g=ablation_energy*ECI

where ablation_energy represents the amount of ablative energy providedat the location through generator 24 and ECI is an index representativeof the degree of electrical coupling between electrode 52 and tissue 12.In another embodiment of the invention, the index CAI may be derivedfrom the following function:

g=α*ablation_energy*ECI+β*temperature

where α and β are constants and temperature represents the temperatureproximate the distal end 50 of catheter 16. Each factor in the index CAImay be compared to a threshold value associated with the factor toensure that any factor forming part of the index CAI is indicative of apredetermined level of ablation therapy at the location (e.g., therapythat will result in irreversible lesions as opposed to brief andreversible exposure of the tissue to ablation). For example, the degreeof coupling between electrode 52 and tissue 12 input to the index CAImay be computed as follows:

ECI=ECI(t)−ECI_(non-contact)

where ECI_(non-contact) represents a threshold value for ECI (typically100+/−5, whereas ECI is typically between 140 and 180 when in electrode52 is in moderate contact with tissue 12). Similarly, ablation_energymay be calculated relative to a threshold value intended to compensatefor the fact that a very low power level (e.g., 1 watt) will not resultin detectable changes to tissue 12:

ablation_energy=ablation_energy(t)−1

Because measured temperatures above the internal body temperature areindicative of effective ablation therapy, a function for determining atemperature value in the index CAI may use body temperature as athreshold value:

Temperature=Catheter_temperature(t)−Body_temperature(t)

It should be understood that temperature values may also be impacted byirrigation, the flow rate of irrigation fluid and other designparameters for catheter 16 and that the above function could be variedto account for such parameters.

As noted hereinabove, ECU 32 may determine the index (CAI) relative to adiscrete location, such as location 76, within the volume 74. In oneembodiment of the invention, ECU 32 may further be configured todetermine the location not as a discrete value, but rather in responseto a plurality of measured positions of catheter 16 over a predeterminedperiod of time. ECU 32 may determine a location by taking, for example,a mean or median value from among a plurality of positions of catheter16 measured over a time period (e.g., between 0.2 to 2 seconds). ECU 32may associate the cumulative CAI value over that time period with thedetermined location. This approach reflects the fact that ablativetherapy may be applied to a number of distinct locations that are veryclose to one another-particularly in short interval.

It should be understood that ECU 32 may calculate values for CAI atlocation 76 responsive to measurements made at multiple electrodes(e.g., at electrodes 52, 54, 56). For example, the degree of coupling(e.g., electrical coupling or force of contact) between each electrode52, 54, 56 and tissue 12 can be assessed. Mathematical algorithms (e.g.,weighting and interpolation) may be used in connection with themeasurements made at the electrodes 52, 54, 56 to derive a more preciseassessment of the degree of coupling at location 76 and, consequently, amore precise value of index CAI.

Referring again to FIG. 3, in one embodiment of the invention, ECU 32 isconfigured to divide the volume 74 into a plurality of volume elementsor voxels 78. A data structure is maintained by ECU 32 which correlatesinstantaneous and cumulative values for the index CAI and the factorsused to compute the index with each voxel 78 such that the index CAI andthe factors reflect cumulative values taken at a plurality of discretelocations within each voxel 78. In this embodiment, one factor that maybe used to compute the index comprises the time in which the ablationcatheter 16 is within the voxel as opposed to time spent at a singlediscrete location. This embodiment of the invention is advantageousgiven the limitations on providing precise representations of thesurface of tissue 12 during mapping and the ability to deliver ablationtherapy to discrete locations.

ECU 32 is configured to alter a visual characteristic of voxel 78responsive to the index CAI. In one embodiment of the invention, thevisual characteristic comprises color and, in particular, the intensityof the color. Voxels 78 containing locations 76 where effective ablationtherapy is delivered may assume a certain color that becomes moreintense or more opaque (illustrated in the drawings by an increase incross-hatching) as the effectiveness of the therapy increases (asindicated by index CAI). Voxels 78 containing locations where therapyhas not been provided or therapy has been relatively ineffective may betransparent or translucent to facilitate the visualization of underlyinggeometric structures in tissue 12. It should be noted that the visualcharacteristic does not need to be altered in a linear fashion relativeto changes in the index CAI. Various functions may be applied to theindex CAI to emphasize specific indices or range of indices likely torepresent ineffective ablative therapy. It should also be understoodthat the visual characteristics of voxel 78 could be altered in responseto other factors or measurements in addition to the index CAI. Forexample, the visual characteristics of voxel 78 may be alteredresponsive to a measured temperature exceeding a predetermined thresholdduring ablation such that the appearance of the voxel is altered inmultiple ways (e.g, by adjusting the intensity of the color based onindex CAI and by alternating between multiple states (i.e., flashing) inthe presence of excessive temperatures).

Referring to FIG. 4, in another embodiment of the invention, ECU 32generates an anatomical map 80 that defines a surface 82. ECU 32 isconfigured to alter a visual characteristic of a portion of the surface82 responsive to the index CAI and a distance d between a location 84 atwhich CAI is computed and one or more locations on surface 82 (therelative locations being known by detecting the position of the cathetertip as discussed above and registering the coordinate system with thatof map 80 in a conventional manner). ECU 32 may alter surface 82 byagain adjusting the color, and particularly the intensity of the color(again illustrated by increased cross-hatching), at the surface 82 or bysuperimposing a marker (e.g., a disc) thereon. As discussed above, itshould be understood that the visual characteristics of surface 82 mayalso be altered responsive to other factors or measurements in additionto the index CAI.

Referring again to FIG. 1, display device 34 is provided to display map72 or 80 and present information permitting the clinician to assesseffective delivery of ablation therapy to tissue 12. Device 34 may alsoprovide a variety of information relating to visualization, mapping andnavigation as is known in the art including measures of electricalsignals, various two and three dimensional images of the tissue 12 andthree-dimensional reconstructions of the tissue 12. Device 34 may, forexample, display, or combine with map 72 or 80, detailed computedtomography or magnetic resonance images using conventional registrationand fusion processes. Device 34 may comprise an LCD monitor or otherconventional display device.

I/O device 36 is provided to allow the clinician to control operation ofsystem 30. Device 36 may comprise a keyboard, mouse or otherconventional input/output device. In accordance with one aspect of thepresent invention, device 36 permits the clinician to exercise a measureof control over the computation of index CAI to permit clinicians toprovide appropriate weight to those factors the clinician believes arebetter indicators of effective ablation therapy. ECU 32 is thereforeconfigured to receive a user input through device 36 controlling aweight accorded to at least one of the factors used by ECU to deriveindex CAI. In this manner, the clinician can, for example, more easilycontrol lesion formation in sensitive areas (e.g., in cardiac tissuenear the esophagus).

The above description contemplates review of the alteredelectroanatomical map 72 or 80 and appropriate adjustment of componentsof system 10 (e.g., by movement of catheter 16) in response thereto. Inan alternative embodiment of the invention, catheter 16 may becontrolled automatically (e.g., robotically or magnetically) responsive,in part, to the measured values of CAI. System 30 may obtainmeasurements and calculate the index CAI. The location of the catheter16 may be adjusted automatically responsive to the values of CAI using,for example, the robotic or magnetic systems described in commonlyassigned U.S. patent application Ser. No. 12/622,488, the entiredisclosure of which is incorporated herein by reference. As part of thisprocess, system 30 may identify locations within volume 74 (by alteringthe visual characteristics of map 72 or 80) that require review, andpossibly further treatment, by a clinician.

Referring now to FIG. 5, a series of timing diagrams (in registrationwith each other) taken during an animal study illustrate changing valuesover time for several individual factors indicative of the delivery ofablation therapy by a tip electrode 52 at two locations (x₁, y₁, z₁) and(x₂, y₂, z₂) within cardiac tissue. The first location (x₁, y₁, z₁) wasin the left atrium while the second location (x₂, y₂, z₂) was in theright atrium. The catheter tip was maintained out of contact with thetissue when electrode 52 was at the first location (x₁, y₁, z₁) and inmoderate contact with the tissue when electrode 52 was at the secondlocation (x₂, y₂, z₂). The power level was varied at each locationbetween 1 watt, 20 watts and 40 watts as illustrated in the timingdiagram marked “ABL POWER”. Although the amount of ablative energydelivered in each location was the same, the catheter tip temperature,the electrical coupling index (ECI) (as well as its individual componentparts R1 and X1) and other values remained relatively constant over thetime period in which the electrode 52 was at the first location (x₁, y₁,z₁) and varied considerably during the timer period in which theelectrode 52 was the second location (x₂, y₂, z₂). Similarly, the indexCAI remained relatively constant during the time period the electrode 52was at the first location (x₁, y₁, z₁) and varied during the time periodin which the electrode 52 was at the second location (x₂, y₂, z₂) toindicate more effective delivery of ablation therapy. In the illustratedembodiment, CAI was calculated from the following equation:

CAI=0.025*(ECI−90)*(ablation_energy−3.4)+2*(temperature−37)

With ECI, ablation_energy, and temperature all compared to thresholdvalues such that each factor impacts CAI only if it is indicative of apredetermined level of ablation therapy at the location (e.g., therapythat will result in irreversible lesions as opposed to brief andreversible exposure of the tissue to ablation).

In summary, the effective delivery of ablation therapy to tissue 12 isassessed through several method steps in accordance with one embodimentof the present invention. First, a three-dimensional anatomical map 72or 80 of the tissue 12 is generated. The map may be generated by ECU 32in a conventional manner using values measured from the movement of aconventional mapping catheter or may be based on a real-time image suchas an ultrasound image or fluoroscopic image or a pre-acquired image(e.g., an image generated by an MRI, CT, ultrasound or fluoroscopicsystem). The map 72 or 80 is then displayed on, for example, aconventional display device 34. ECU 32 then generates an index CAIcorresponding to a location 76 or 84 within a volume 74 or 82 defined bythe map 72 or 80, respectively. The index CAI is indicative of a stateof ablation therapy at the location. Finally, a visual characteristic ofa portion of the map 72 or 80 corresponding to the location 76 or 84 isaltered responsive to the index.

A system and method in accordance with the present teachings offers oneor more of a number of advantages. The system and method provide a lesssubjective and more accurate map of ablation lesions than conventionalsystems and methods. The use of an index based on one or more factors orpredictors indicative of the state of ablative therapy provides a moreobjective assessment of the effective delivery of ablation therapy whilereducing the impact of variabilities such as catheter or tissue movementor changes in operating conditions. As a result, clinicians can assessthe effectiveness of ablation therapy with greater confidence and reduceor eliminate the need for electrophysiologic mapping procedures andrepeated ablation.

Although several embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art couldmake numerous alterations to the disclosed embodiments without departingfrom the scope of this invention. All directional references (e.g.,upper, lower, upward, downward, left, right, leftward, rightward, top,bottom, above, below, vertical, horizontal, clockwise andcounterclockwise) are only used for identification purposes to aid thereader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Joinder references (e.g., attached, coupled, connected, andthe like) are to be construed broadly and may include intermediatemembers between a connection of elements and relative movement betweenelements. As such, joinder references do not necessarily infer that twoelements are directly connected and in fixed relation to each other. Itis intended that all matter contained in the above description or shownin the accompanying drawings shall be interpreted as illustrative onlyand not as limiting. Changes in detail or structure may be made withoutdeparting from the invention as defined in the appended claims.

1. (canceled)
 2. A system for assessing effective delivery of ablationtherapy to a tissue in a body, comprising: an electronic control unitconfigured to generate a three-dimensional anatomical map of saidtissue, said map defining a corresponding volume; and a displayconfigured to display said anatomical map; wherein said electroniccontrol unit is further configured to generate an index corresponding toa first location within said volume, said index indicative of a state ofablation therapy at said first location, and to alter, responsive tosaid index, a visual characteristic of a portion of said anatomical mapcorresponding to said first location; and wherein said index isgenerated responsive to a detected position of a catheter ablationelectrode relative to said anatomical map only when a value of saiddetected position is less than a threshold value associated with saiddetected position.
 3. The system of claim 2, wherein said volume isdivided into a plurality of voxels and said portion of said anatomicalmap comprises a first voxel of said plurality of voxels, said firstlocation within said first voxel.
 4. The system of claim 2, wherein saidvisual characteristic comprises a color.
 5. The system of claim 4,wherein said visual characteristic comprises an intensity of said color.6. The system of claim 2, wherein said electronic control unit isfurther configured to adjust a weight accorded to said detected positionresponsive to a user input.
 7. The system of claim 2, wherein said indexis also generated responsive to a duration of time during which ablationenergy is provided at said first location.
 8. The system of claim 2,wherein said index is also generated responsive to an amount of ablationenergy provided at said first location.
 9. The system of claim 2,wherein said index is also generated responsive to a degree of contactbetween said ablation electrode and said tissue.
 10. The system of claim2, wherein said index is also generated responsive to a temperatureproximate said first location.
 11. The system of claim 2, wherein saidindex is also generated responsive to a change in amplitude in anelectrogram.
 12. The system of claim 2, wherein said index is alsogenerated responsive to a flow rate of an irrigation fluid in anablation catheter on which said ablation electrode is disposed.
 13. Thesystem of claim 2, wherein said index is also generated responsive to anorientation of said ablation electrode relative to said tissue.
 14. Thesystem of claim 2, wherein said index is also generated responsive to animpedance measurement proximate a distal end of said ablation electrode.15. The system of claim 2, wherein said index is also generatedresponsive to a degree of coupling between an ablation electrode andsaid tissue.
 16. The system of claim 15, wherein said degree of couplingis a degree of physical coupling between said ablation electrode andsaid tissue.
 17. The system of claim 15, wherein said degree of couplingis a degree of electrical coupling between said ablation electrode andsaid tissue.
 18. The system of claim 2, wherein said index is alsogenerated in response to (a) an amount of ablation energy provided atsaid first location and (b) a temperature proximate said first location.19. The system of claim 2, wherein said electronic control unit isfurther configured to generate said index responsive to first and secondvalues of a second factor indicative of the state of ablation therapy atsaid first location, said first value of said second factor obtained atsaid first time and said second value of said second factor obtained atsaid second time.
 20. A method for assessing effective delivery ofablation therapy to a tissue in a body, comprising: generating athree-dimensional anatomical map of said tissue, said map defining acorresponding volume; displaying said anatomical map; analyzing adetected position of a catheter tip relative to said anatomical map,wherein said detected position is indicative of a state of ablationtherapy at a first location within said volume; comparing said detectedposition against an associated threshold value; updating an indexcorresponding to the first location within said volume if said detectedposition is less than the associated threshold value; and altering,responsive to said index, a visual characteristic of a portion of saidanatomical map corresponding to said first location.
 21. The method ofclaim 20 further comprising the step of adjusting a weight accorded tosaid detected position responsive to a user input.
 22. The method ofclaim 20, wherein said index is also updated responsive to an amount ofablation energy provided at said first location.
 23. The method of claim20, wherein said index is also updated responsive to a degree of contactbetween said catheter tip and said tissue.
 24. The method of claim 20,wherein said index is also updated responsive to a degree of couplingbetween said catheter tip and said tissue.
 25. The method of claim 24,wherein said degree of coupling is a degree of physical coupling betweensaid catheter tip and said tissue.
 26. The method of claim 24, whereinsaid degree of coupling is a degree of electrical coupling between saidcatheter tip and said tissue.
 27. The method of claim 20, wherein saidindex is also updated responsive to an electrical coupling index (ECI).28. The method of claim 20, wherein said index is also updatedresponsive to a temperature proximate a tip of an ablation catheter.